1. Field of the Invention
The present invention relates to communications systems and is useful in particular for mobile telephone systems which exchange code division multiple accessor (CDMA) transmit and receive signals between a mobile handset and a base station through a signal conduit, (e.g. a co-axial cable or an optical fiber cable in a cable television (CATV) plant), or via dedicated co-axial cable of a MEX distributed antenna array.
2. Description of the Related Art
It is known to provide telephone communication to a mobile telephone handset by collecting one or more RF repeaters in the form of commercially available remote antenna drivers (RADs) through a CATV plant to a remote antenna signal processor (RASP) and a base station. The base station interfaces with a public switched telephone network and provides modulated transmit signals, e.g. in the form of code division multiple access (CDMA) transmit signals, through a series combination of RASP-CATV plant-RADs for transmission as radio signals to the mobile handset. From the mobile handset, signals are transmitted as radio signals to the RADs, from which they are passed through the CATV plant to the RASP, and then to the base station for demodulation and connection to the public switched telephone network.
The base station supplies the transmit signals at a first predetermined frequency to the RASP, at which the transmit signals are frequency converted to a second or intermediate frequency, filtered and again frequency converted to a third frequency, which is suitable for transmission through the CATV plant to the RADs. At the RADs, the transmit signals are frequency converted to an intermediate frequency, filtered and then frequency converted back to the first frequency, at which they are transmitted as radio signals to the mobile handset. Signals from the mobile handset are similarly processed through the RADs-CATV plant-RASP combination so as to be transported back to the base station.
Another feature characteristic of the RAD-RASP technology is the use of simulcast techniques to create large coverage areas from the summation of smaller coverage zones. That is to say that a RASP can support more than one RAD simultaneously, and that those simultaneously supported RADs use the same frequencies for transmission of signals over the CATV plant, (both RASP-to-RAD and RAD-to-RASP transmissions), and as radio signals (i.e. RAD to mobile handset and mobile handset to RAD).
This simulcast technique allows many RADs to operate without demanding excessive bandwidth from the CATV plant.
By siting RADs at some distance from one another mobile handsets over a wide area can be supported with implications on the ability to roam within that wide area i.e. the RADs may be located at a multiplicity of remote sites.
The wireless line is typically enhanced by using two (or more) antennas in the locale of the remote site to receive the mobile transmissions. This practice is referred to as antenna 'diversity, and is effective in improving the wireless link quality in certain radio propagation environments (e.g., multipath environments).
Thus, antenna diversity at an individual RAD can be used to offset the effect of path fading that would occur using a single receive antenna. More particularly, the use at the RAD of a second antenna spaced at an optimum distance from the first will provide a second or diversity channel that can be presented to the base station so that the base station can use the best of the two channels, if required. This diversity option has the effect of extending the reliable coverage area. To give a concrete example:
A RAD is the equipment used at the remote site. A RAD would typically be installed 18-25 feet above the ground on the steel wire between two telephone poles. The radio link is a 1.9 GHz CDMA system.
The RAD might have three antennas: one transmit and two receive antennas. These three antennas would typically be spaced apart with three to six feet between each antenna being the norm. Obviously, in practice., the nature of a RAD site can constrain the maximum separation that can be achieved in any event.
The purpose of the additional receiving antenna is to improve the link quality in the presence of multipath: i.e., if one receiving antenna is in a multipath null, it is unlikely the second antenna is simultaneously in a multipath null since the three to six feet separation represents many wavelengths at the operating frequency. (Note there is very little benefit having greater than 10 wavelengths of separation. At 1.9 Ghz, 10 wavelengths is a 5 feet separation.)
The RAD can support many mobile handsets simultaneously. In addition, the demodulation resource is contained within the base station at the central site. These two facts make it difficult for the RAD to determine which receiving antenna is the "best". Indeed, if two mobile handsets are operating, they may give optimal performance each using different antennas. As a consequence, the RAD must be constructed to present both sets of receive signals for the base station to demodulate.
Thus, use of antenna diversity at the RAD results in the use of twice the CATV plant bandwidth between the RAD and the centralized base station (the "upstream" or "receive" signal path), i.e. to allow the benefits of antenna diversity the base station needs independent access to the receive signal waveforms at both receiving antennas at the RAD.
The doubling of return path CATV bandwidth is a distinct disadvantage in the use of RAD-RASP equipment, since the CATV plant typically has a restricted amount of return path bandwidth available. In addition, increased cost and complexity are inevitable once an independent return path is added.
Other concrete examples could be given for RADs using different numbers of antennas, or of using antenna diversity in the improvement of signal quality at the mobile handset (i.e. antenna diversity for RAD transmissions), with similar conclusions that increased CATV plant bandwidth is required.
In describing the related art it is important to note the commercially available MEX (microcell extender) technology. This technology allows the same functions to be performed as the RAD-RASP, but utilizes dedicated coax between remote sites (at which MEX devices, not RADs, are located). Consequently the advantages and disadvantages of antenna diversity are the same for both RAD and MEX technologies. The term "RF repeater" is therefore used herein to include both RADs and MEXs.
Certain CDMA wireless technologies employ a RAKE receiver in the base station, and/or in the mobile handset.
A RAKE receiver allows the received signal to be distinguished not just by virtue of its operating frequency, its time of operation and its code, but also by the net delay incurred between the transmitter and the receiver. More precisely, the RAKE can distinguish between signals of the same code and nominally the same signal strength, but of different delay, provided the delay is equal to or larger than a chip period.
IEEE Communications Magazine, Dec. 1992 ("Implications of Mobile Cellular CDMA by A. J. Viterbi et al.), (see FIG. 8 in particular) exemplifies the ability of the RAKE to distinguish the mobile handset signals via a path delay.